The present invention relates to a wavefront aberration measurement method for a projection optical system or the like, which is used in a lithography step of manufacturing a semiconductor element and transfers a mask pattern onto a photosensitive substrate, and a projection exposure apparatus using the method.
The manufacture of a semiconductor element or the like by photolithography uses a projection exposure apparatus for transferring a circuit pattern or the like on a reticle or photomask (to be generally referred to as a reticle hereinafter) to a semiconductor wafer (photosensitive substrate) or the like coated with a photosensitive agent. A projection exposure apparatus of this type must accurately transfer a reticle pattern to a wafer at a predetermined magnification (reduction ratio). To meet this demand, the projection exposure apparatus must exploit a projection optical system which exhibits good imaging performance with little aberration. In recent years, a pattern exceeding the general imaging performance of an optical system is often transferred along with further miniaturization of a semiconductor device. The transfer pattern, therefore, is more sensitive to the aberration of the optical system. On the other hand, the projection optical system must increase the exposure area and numerical aperture (NA), which makes satisfactory aberration correction more difficult.
In this situation, demands are arising for measuring aberration, particularly, wavefront aberration of a projection optical system while the projection optical system is mounted in a projection exposure apparatus, i.e., is actually used for exposure. This enables more precise adjustment of lenses (optical system) corresponding to the use state and device design almost free from the influence of aberration. To meet these demands, the image intensity distribution is measured by a knife edge or slit, as a conventional means of obtaining the imaging performance of a projection optical system while the projection optical system is mounted in the exposure apparatus. Also, the contrast of a pattern having a specific shape such as a bar chart is obtained. However, these methods do not attain wavefront aberration. In some cases, an interferometer is used as a method of obtaining wavefront aberration. The interferometer is generally used as an inspection device in the manufacture of a projection optical system, and is not practically mounted in a projection exposure apparatus in terms of the technique and cost.
To measure the aberration of a projection optical system in lithography of manufacturing a semiconductor element by a method of obtaining an image intensity distribution by using a knife edge or slit, the S/N ratio in intensity distribution measurement must be about 106 or more, which is very difficult to attain. To measure wavefront aberration by a method of obtaining a contrast by using a bar chart, the contrasts of many bar charts must be attained from a rough pitch to a pitch exceeding the resolution limit, which is not practical in terms of formation of bar charts and the measurement labor.
In the wavefront aberration measurement method using an interferometer, an interferometer made up of a prism, mirror, lens, and the like, and an interferometer illumination system with good coherence must be arranged near a reticle stage or wafer stage. In general, the space near the wafer stage or reticle stage is limited, and the sizes of the interferometer and illumination system are also limited. Limitations are also imposed in terms of heat generation and vibrations, and the interferometer is difficult to mount. With recent decreases in exposure wavelength, an interferometer light source with good coherence in the exposure wavelength region does not exist or is very expensive. Thus, it is impractical in terms of the technique to mount an interferometer type aberration measurement device in a projection exposure apparatus.
It is an object of the present invention to provide a wavefront aberration measurement method capable of easily measuring the imaging performance, particularly, wavefront aberration of a projection optical system in a projection exposure apparatus, and a projection exposure apparatus using the method.
It is another object of the present invention to provide a wavefront aberration measurement method capable of easily measuring the wavefront aberration of a projection optical system in an actual projection state in an apparatus for projecting a reticle pattern onto a wafer, and a projection exposure apparatus using the method.
According to the present invention, the foregoing object is attained by providing a wavefront aberration measurement method of measuring wavefront aberration of a projection optical system in a projection exposure apparatus for projecting a reticle pattern onto a substrate via the projection optical system, comprising: the measurement step of measuring a light intensity of an image formed by a beam having passed through a phase shift pattern when the phase shift pattern set near an image plane or object plane of the projection optical system for forming an image of a pattern is scanned in one or a plurality of directions perpendicular to an optical axis of the projection optical system; and the signal processing step of calculating the wavefront aberration of the projection optical system on the basis of a measurement result of the measurement step.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.